ACTIVATED ALUMINA : HEATOF WETTINGBY HYDROCARBONS
April, 1952
487
ACTIVATED ALUMINA: HEAT O F WETTING BY HYDROCARBONS BY VERNONM. STOWE Aluminum Reaearch Laboratories, East St. Louis, IllinOis Received M a y ‘7, 1861
The heat of wetting of activated alumina Grade XF-21 by four hydrocarbons containing 7 carbon atoms was investigated. The heat of wetting by n-heptane and triptane was about 2.7 cal. and by methylcyclohexane and toluene was 4.1 to 4.7 cal. per gram of alumina. When water in excess was added to alumina containing adsorbed aliphatic and cyclic hydrocarbons the heat relations indicated relatively complete displacement. However, when a cyclic hydrocarbon was added in excess to alumina containing an adsorbed aliphatic, there was no thermal evidence of a displacement. The data explain the performance of activated alumina in drying natural gas containing heavier hydrocarbons. The method offers a good approach to other problems of preferential adsorption.
Preferential adsorption is a fascinating field with many important commercial applications, including the drying of natural gas, drying of liquid hydrocarbons and refrigerants, chromatographic analysis, displacement of petroleum from oil-bearing sands by injection of water, and many others. I n the work reported here, the heat of wetting of activated alumna by some hydrocarbons as well as by water was measured, to determine t o what extent such data might form a basis for correlating the behavior of adsorbents in preferential adsorption.
temperature by introducing more of the given hydrocarbon, precooled to 0’ in the ice-jacketed buret.
Experimental Results and Discussion The differential heat of wetting of activated alumina Grade XF-21 by the hydrocarbons studied, each containing 7 carbon atoms, is shown in Fig. 1. The data of Fig. 1 are represented by three smoothed curves. A point of inflection may exist between 2 and 4 g. of hydrocarbon per 100 g., of adsorbent, however.
Materials and Procedure The apparatus and the adsorbent employed (activated alumina Grade XF-21 produced by the Aluminum Company of America) are described in another paper.’ I n the freshly reactivated form it contained 8.7% water of which most i s combined water. n-Heptane from Westvaco Chlorine Products Company was used. This was stated to have a boilinggoint of 98.44”, melting point minus 90.66”, density a t 20 f 0.68382 g. per ml., and a refractive index of 1.38779. %he triptane (trimethylbutane) was 99 % material donated by the Redford Laboratory of the General Motors Corp. The methylcyclohexane was obtained from Eastman Kodak Co., and was “practical grade.” The toluene was obtained from Fisher Scientific Co., C.P. grade. Heat capacities per milliliter were calculated from the data of M. P. Doss.2
+
TABLE I VOLUMETRIC HEATCAPACITY OF Hydrocarbon
Specific heat, cal./°C./g.
a-Heptane Triptane Methylcyclohexane Toluene
0.525 .497 .443 .404
THE
HYDROCARBONS
Specific gravity, g./ml.
0.684
.690 .769 .867
Volumetric heat ca acity, cal./08./m~.
0.359 .343 .341 .350
To prepare samples for study of the heat of wetting, 25
X 200-mm. test-tubes of Pyrex glass were constricted near
the mouth, and weighed portions of adsorbent were introduced. The desired amount of adsorbate was introduced by means of a pipet. After withdrawing the pipet, the tube was turned so as to cover t,he wetted surface with fresh dry adsorbent, restricting losses by evaporat,ion. The weight of adsorbate added was determined and the tube was sealed a t the point of constriction. After cooling, the tube was turned about to ensure uniform mixture. I n no case was enough liquid used to cause the particles to cling toget,her atter initial mixing had been effected. The contents of the tubes were stirred by turning about from time to time. At least one week was allowed for the material to come to a uniform adsorption throughout the tube. To make a determination, the contents of a tube were transferred t,o the dewar flask and an excess of the given hydroca1,bon was added. The temperature was then reduced to the starting (1) V. M. Stowe, THISJOURNAL, 66, 484 (1952). (2) M. P. Dose, “Physical Constants of the Principal Ifydi,noarbons,” The Texas Company, New York, N. T.,1943.
Fig. 1.-Heat
Hydrocarbon already adsorbed, yo. of wetting activated alumina grade XF-21 by various hydrocarbons.
Wetting by the hydrocarbons investigated generates much less heat on activated alumina than does wetting by water. The maximum heat evolved with normal heptane was only 2.7 cal. per gram of freshly reactivated adsorbent. Triptane caused the evolution of 2.6 cal. per gram. Data for these compounds are represented by a single line in Fig. 1. The ring compounds evolve nearly twice as much heat of wetting as do the aliphatic hydrocarbons tested, namely, 4.7 cal. per gram of adsorbent for methylcyclohexane and 4.1 cal. per gram of adsorbent for toluene. Figure 1 shows that heat is being evolved even when 10 g. of the given hydrocarbon per 100 g. of adsorbent had already been adsorbed. For a calculation of the heat of wetting by a monolayer of heptane derived from liquid heptane, the datum of Loeser and H a r k i d for the cross sec(3) E. H. Loeser and Harkina, J. Am. Chem. Soc., 78, 3427 (1950).
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VERNONM. STOWE
Vol. 56
tional area of the adsorbed heptane molecule was per gram of adsorbent. This is in good agreement From this, the factor for conver- with the measured heat of wetting by heptane, ie., used, i.e., 64 sion of weight of heptane, g. per 100 g. of adsorb- 2.7 cal. per gram, and indicates that the water reent, to surface area, square meters per gram, is placed essentially all of the adsorbed heptane. This taken t o be 38.5. Since the surface area of XF-21 behavior is in harmony with the performance shown is 155 sq. meters per gram, the weight of heptane by Ku, el aLj6 who found that when natural gas required for a monolayer is 4.03 g. per 100 g. of the containing water and also hexane, heptane or even alumina. Bare alumina evolves 2.7 cal. per gram butane was passed over activated alumina, substanof adsorbent when wet by normal heptane, and tially all the hydrocarbon and the water was adfrom the curve of Fig. 1 it is estimated that 100 g. of sorbed until about 12% of the combined materials alumina, on which 4.03 g. of heptane is already ad- had been taken up. Thereafter, for a time, subsorbed, would evolve 1.0 cal. per g. on wetting with stantially all the water continued to be adsorbed but heptane. Hence, t o cause a monolayer of heptane no more hydrocarbon was adsorbed. Finally, as to be adsorbed on the activated alumina from liquid more water was taken up, the hydrocarbon content phase would result in the evolution of only 1.7 cal. decreased until, when the alumina became satuper gram of adsorbent, or 42 cal. per gram of hep- rated with water, practically no hydrocarbon retane. It is interesting to note that this quantity of mained on the alumina. heat exceeds the heat of fusion of heptane, ie., 33 Another type of experiment was undertaken in cal. per g.* Cross sectional data were not at hand which water was added in excess to alumina already for similar calculations on triptane, methylcyclo- containing various limited amounts of adsorbed hyhexane and toluene. drocarbon, Le., up to about 501, of methylcyclohexA study was made of the behavior of aluminas ane. The data are shown in Table 111. thoroughly wet with heptane when water was TABLE 111 stirred in with the mixture. The heavier water BY WATER OF GRADE XF-21 CONTAINING soon replaced the excess heptane which rose t o the HEATOF WETTINQ ADSORBED METEYLCYCLOHEXANE surface. The thermal effects shown in Table I1 also indicate that it displaced the adsorbed hep- Parts adsorbed per 100 parts tane. original adsorbent 0 1.62 2.29 4.67 TABLE I1 Heat evolved, cal. HEATEVOLVED WHENACTIVATED ALUMINA GRADEXF-21 Wt. of adsorbent used, g. , ALREADY WETWITH R-HEPTANE IN EXCESS WASTREATXD Cal. per g. adsorbent WITH WATER IN EXCESS Heat evolved by prior adsorption Wt. of adsorbent (fresh), g. 34.7 29.9 32.3 32.2 of methylciclohexane, cal. per Heat evolved, cal. 413 329 381 354 g. adsorbent (from Fig. 1) Cal. per gram adsorbent 11.9 11.3 11.8 11.0 Calculated heat of wetting by water, cal. per g. adsorbent From Table 11, it appears that this operation re-
leased on the average 11.5 cal. of heat per gram of original sorbent. As reported in the previous paper, the heat evolved when fresh activated alumina is wet with water is 14.1 cal. per gram. The heat resulting from the prior wetting by heptane may be taken as the difference, 14.1 minus 11.5 or 2.6 cal.
Parti of n-heptane or methylcyclohexane per 100 parts of alumina. Fig. 2.-Heat of wetting activated alumina grade XF-21 by methylcyclohexane. (4)
K. 8. Pitrer, J . Am. Chem. Boo.,
62, 1224 (1940).
1345 466 436 393 92.2 34.0 32.8 32.7 14.1 13.7 13.3 12.0
..
1.5
1.9
2.6
14.1 15.2 15.2 14.6
The calculations involved in Table I11 will be illustrated using the second column of data. The heat of wetting of bare alumina by methylcyclohexane is 4.7 cal. per gram of adsorbent (Fig. 1). From the curve plotted in Fig. 1 the heat of wetting by methylcyclohexane of alumina already containing 1.62 parts of adsorbed methylcyclohexane per hundred parts alumina is 3.2 cal. per gram. Hence the adsorption of 1.62 parts of methylcyclohexane from liquid phase caused the evolution of 4.7 minus 3.2 or 1.5 cal. per gram of alumina. The 1.5 cal. supplied for the desorption of the hydrocarbon is added to the observed value 13.7 giving 15.2 cal. per gram, the calculated heat of wetting of bare alumina by water. That the calculated heat of wetting lies somewhat above the actual measured heat of wetting by water (14.1 cal. per gram) may be due to experimental error. At any rate, complete replacement of the previously adsorbed methylcyclohexane is indicated. I n one other type of experiment, the heat of wetting by excess methylcyclohexane of alumina already containing various limited amounts of adsorbed heptane was tested. The results are shown in Table IV and in Fig. 2. Since the heat of wetting of alumina by methylcyclohexaneis about twice as great as by n-heptane, (6) C. P. Ku, R. L. Huntington and L. 8. Reid. Am. I n s l . Mining Met. Enoro., Tech. Publ. No. 1628 (1943).
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April, 1952
Avzanami POLYMER MOLECULAR WEIQHT
489
than for samples containing equal weights of methTABLE IV ylcyclohexane, provided the excess methylcyclohexDIFFERENTIAL HZATOF WETTINGBY METHYLCTCLOHEX~PTE OF ACTIVATED ALUMINAGRADEXF-21 CONTAININ~ (ZERO ane is able to displace adsorbed n-heptane. However, it can be seen in Fig. 2 that the heat of wetting TO ABOUT 10 Q.) ADSORBED *HEPTANE(PER 100 Q. of activated alumina Grade XF-21, containing adsorbed +heptane is about equal to that of alumina containing an equal weight of adsorbed methylcyclohexane (for which hydrocarbon the data are reproduced in Fig, 2 for convenient comparison). It is concluded that methylcyclohexane will not replace normal heptane from adsorption under these conditions. Probably more “drive” than 2 cal. per gram of adsorbent is required t o effect displace treating these samples with methylcyclohexane ment within the time involved in this type of experishould result in somewhat greater heat of wetting ment.
ADSORBZNT) Parts heptane adsorbed 0 0.95 1.92 4.10 9.65 per 100 parts adsorbent Heat evolved on adding methylcyclohexane,cal. 99 115 95 74 45 Wt. of adsorbent, g. 21.0 86.0 33.8 32.7 35.4 4.7 3 . 2 2 . 8 2 . 3 1 . 3 Cal. per g. adsorbent
ON THE DEPENDENCE OF THE AVERAGE POLYMER MOLECULAR WEIGHT ON THE CHANGE IN THE PARTICLE DIAMETER OF A HOMOGENEOUS MONOMER EMULSION BY G. NARSIMHAN Department of Chemical Engineering, Laxminarayan Inrfitute of Technology, Nagpur Univerrity, Nagpur, India Recritad May 17, io61
Two new equations are proposed for the determination of the average polymer molecular weight during the polymerization of a soap stabilized monomer emulsion. The derivation is based on the assumptions that the polymerization proceeds at the interface according to the limiting value of the Boltzmann distribution function and that the emulsion possesses a homogeneOUE radius distribution. The initiation and growth of the chain a t the active centers is also assumed.
Emulsion polymerization has been assumed to proceed at the monomer interface by few workers. Even in the absence of micellar soap and catalyst, polymerization has been found t o take place but slowly a t the aqueous phase.’ The initiation and progress of such a reaction is attributed t o the diffusion of monomer molecules through the interfacial layer of soap into a few activated molecules present in the aqueous phase and the gradual growing of this complex as more molecules diffuse. On the other hand, Montroll* assumes polymerization to initiate and terminate at the emulsion interface for non-micellar soap and a water-soluble catalyst. The polymerization starts according to him, after the natural inhibitor concentration falls to an optimum value at the interface by the diffusion of the inhibitor molecules through the interface into the aqueous phase consequent on the disturbed equilibrium arising out of the reaction of the catalyst with the inhibitor present in the aqueous phase. For a homogeneous radius distribution, the reaction starts suddenly at the end of an initiation period and the extent of the reaction has been found t o be a parabolic function of the polymerization time. First Order Reaction.-Norrish’ investigated the catalyzed polymerization of styrene and methacrylate and it was found that the active centers grow into long chains according t o a zero order reaction and that the molecular weight of the polynier obtained was found to be proportional to the time of reaction and inversely so to the catalyst concentra-
tion. No general correlation has, till now, been possible between the average polymer molecular weight and the varying diameter for a given system and polymerization time, for interfacial polymerization. * I n the absence of any thermodynamic relation specifying the extent of reaction among the interfacial molecules in terms of physico-chemical constants, a general and rough relation has been derived for the dependence of the average polymer molecular weight on the change in the particle diameter of a soap stabilized monomer emulsion based on the following assumptions: (1) The reaction takes place at the boundary of the suspended spheres. (2) The reaction starts after the inhibitor concentration’ has fallen to an optimum value. (3) The interfacial molecules possess a few active centers (constant and proportional to catalyst concentration). (4) The reaction is initiated at these active centers and progresses according to Boltzmann’s distribution law, the most frequent degree of polymerization being obtained based on the limiting value of the distribution function. ( 5 ) The interfacial polymerization stops after the reaction has progressed according to assumption 4. The reaction is again initiated only after the ejection of the polymer particle into the aqueous phase and the subsequent interfacial adsorption of non-micellar soap from the aqueous phase. This assumption is in agreement with the observed experimental fact that as polymerization proceeds, soap disappears from solution.’
(1) W. D.Harking, J. Polvrnsr Sei., 6, 233 (1950). (2) E. W. Montroll, J . Chsm. Phyr., 18, 337 (1846). (3) R.0. W.Norrish and E.F.Brookmen, Proc. Roy. Soc. (London), A f l l , 147 (1839).
Derivation This is based on the assumption that interfacial polymerization is governed by the Boltzmann dis-
